1. Field of the Invention
The present invention relates to a light emitting sheet module which is installed in an operation panel of each of various electronic instruments and has an illumination function.
2. Description of Related Art
A sheet switch 10 as shown in FIG. 13 is used as a key inputting part installed in each of various electronic instruments such as mobile phones, mobile information terminals or the like. The sheet switch 10 achieves its illumination function through provision of a plurality of light sources including light emitting diode elements 6, each of which is disposed about each of key switches 8.
FIG. 14 illustrates a sheet switch 1 including light emitting elements 6 and a flat plate-like light guiding plate 5 (for reference, see Japanese Patent Application Publication No. 2004-69751, FIG. 9). The sheet switch 1 includes a circuit board 2 provided with a wiring pattern, fixed contacts 3 arranged on the circuit board 2, dome contacts 4 each of which works as a tact spring and is disposed on the circuit board 2 to cover each of the fixed contacts 3, the light guiding plate 5 disposed above the circuit board 2, a light emitting diode 6 (hereinafter, referred to as light emitting diode, or LED) which is a light emitting diode element or package including at least one light emitting diode element configured to illuminate one side surface of the light guiding plate 5, and key tops 7 each at the other side including an extended lower end portion which is configured to extend through the light guiding plate 5 and press each of the dome contacts 4. Light emitted from the LEDs 6 and guided in the light guiding plate 5, finally illuminates the lower end portion of each of the key tops 7 and then, the light illuminates the entire of the key tops 7.
Also known is a sheet switch which has been further thinned while retaining an illumination function for meeting a recent trend of thinning electronic instruments, (for reference, see Japanese Patent Application Publication No 2004-69751, FIG. 6). As shown in FIG. 15, the sheet switch 11 includes a circuit board 12 provided with fixed contacts 13 and dome contacts 14 each of which is disposed on the circuit board 12 to cover each of the fixed contacts 13, a light guiding plate 15 covering the area above the dome contacts 14 and the circuit board 13, and key tops 17 provided above the light guiding plate 5. Each of the key tops 17 corresponds to each of the dome contacts 14. In the sheet switch 11, light emitted from the LED 16 disposed on one end of the circuit board 12 is guided into the light guiding plate 15 to provide light emission to the entire light guiding plate 15 and illuminate the key tops 17 from below.
However, the above-mentioned conventional sheet switches face the following problems.
That is to say, in the sheet switch 10 as shown in FIG. 13, in which LEDs 6 are disposed adjacent to the key switches 8, the number of LEDs 6 must be increased as the number of key switches 8 increases. Consequently, the problem arises that power consumption is increased and the sheet switch 10 becomes large in size.
In the sheet switch 1 as shown in FIG. 14, because the light is guided to the key tops 7 through the light guiding plate 5, the number of LEDs is reduced, but in order to guide the light emitted from the LEDs 6 to the light guiding plate 5 efficiently and without leakage, a light guiding plate 5 having a certain thickness must be used. The light guiding plate 5 increasing in thickness results in increased thickness of the entire sheet switch 1.
In the sheet switch 11 as shown in FIG. 15, because the light emitted from the LED 16 enters one side surface of the light guiding plate 15 to illuminate the entire light guiding plate 15, there is a problem that brightness of the key tops at positions remote from the LED 16 is reduced and variations in brightness are noticeable throughout the entire light guiding plate 15.
Because the light guiding plate 15 is merely positioned over the circuit board 12, it is easy for a clearance to occur between the light guiding plate 15 and the sheet switch, in addition, it is easy for a clearance to occur even between the LED and the light guiding plate 15.
Accordingly, if such a clearance exists around the light guiding plate 15, there is a problem that light may leak from the clearance so that an insufficient amount of light passes through the key tops 17 and variations in brightness of the sheet switch occur.
In addition, because each of the above-mentioned sheet switches 10 and 11 including an illumination device comprising the light guiding plate and the LEDs corresponding to the key switches are integrated, it is not possible to utilize only the illumination device function. Therefore, there is a problem that the sheet switches can not be adapted flexibly to various devices in which structures of the key switches, or switch board or the like have different structures.